The present invention relates to lube oil manufacture and more specifically, is directed to increased production of Bright Stock raffinate oil.
Bright Stock raffinate oil is a high-boiling hydrocarbon fraction produced during lube oil manufacture and is well-known in the art. Generally, in the art, after a vacuum residuum, suitable for lube oil production, has been deasphalted by contacting and extracting with a propane solvent, the propane solvent is stripped off and the resulting extract oil is extracted with an aromatic solvent, such as N-methylpyrrolidone (NMP), to remove aromatic hydrocarbons. The resulting extracted, deasphalted oil phase, containing mainly saturated hydrocarbons, is known in the trade as "Bright Stock raffinate oil". This raffinate is subsequently contacted with extracting agents like ketones or alkane hydrocarbon solvents to remove waxy hydrocarbons which are detrimental to lube oil viscosity properties and then subjected to HYDROFINING.TM. improve the appearance of the oil and remove sulfur. The resulting oil is termed "Bright Stock Oil" and is a lubes base stock component from which many lubricating oils, e.g., gear oil, machine oil, automobile engine oil, are made by blending with other lube stock components.
A current practice in the industry, following NMP extraction of deasphalted oil in a lubes process, is to strip off the NMP from the extracted phase and send the residual extract oil, containing mostly aromatics and some dissolved recoverable Bright Stock raffinate oil, to a different process involving a catalytic cracker unit for processing to produce fuel oils. Such practice results in a yield debit for Bright Stock raffinate oil in the lube oil process and ultimately the Bright Stock base oil yield. This step is performed primarily because it is believed that recycling said extract oil again through the extraction step would result in a very low recovery of recoverable raffinate which may not be justified in light of the attendant recycling cost and process time required.
An example of the prior art in the area is U.S. Pat. No. 2,570,044 which discloses recycling of an aromatic extract oil stream derived from a deasphalted oil to a deasphalter feed during lubes production. This is disclosed as being carried out in order to eliminate the formation of a third phase which tends to foul the deasphalter internals.
In addition, the patent claims that the overall yield of high Viscosity Index (VI) components is increased relative to a conventional scheme without extract oil recycle. The aforementioned patent also teaches that the quantity of extract oil recycled to prevent asphaltic deposition is from 3 to 25 percent by volume (LV %) based on the reduced crude oil charged to the deasphalting treater tower. Furthermore, it is stated that recycling larger amounts (greater than 25 LV %) of extract oil is not disadvantageous but actually increases the yield of high Viscosity Index (VI) oil.